Method and apparatus for obtaining synchronization for communication between devices

ABSTRACT

A method and an apparatus for obtaining synchronization of a terminal for communication between devices when a base station collapses are provided. The method includes, when a preamble signal is not received from the base station, maintaining a first synchronization for a predetermined time using a timer, transmitting a first preamble signal at a first synchronization point, performing a synchronization procedure with at least one neighbor terminal based on the first preamble signal, and, after the synchronization procedure, performing communication with the at least one neighbor terminal. In addition, when a preamble signal is not received from a base station, a first preamble signal from a relevant neighbor terminal is monitored for a predetermined time. When the first preamble signal is not received from the relevant neighbor terminal for the predetermined time, a second preamble signal is transmitted at an arbitrary point.

PRIORITY

This application is a continuation application of a prior applicationSer. No. 13/527,001, filed on Jun. 19, 2012, which claimed the benefitunder 35 U.S.C. §119(a) of a Korean patent application filed in theKorean Intellectual Property Office on Jun. 20, 2011 and assigned SerialNo. 10-2011-0059581, the entire disclosure of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system. Moreparticularly, the present invention relates to a method and an apparatusfor obtaining synchronization for direct communication between devices.

2. Description of the Related Art

A communication company establishes a communication infrastructure toprovide various communication services to users. For example, a mobilecommunication company establishes a plurality of base stations inside aservice area to allow a User Equipment (UE) to be connected to one ofthe base stations and receive a service. For example, when a first UEcommunicates with another UE, the communication is made through thecommunication infrastructure, such as a base station.

However, there may be times when one of various problems is generated inthe infrastructure (for example, a base station) such that theinfrastructure no longer operates properly. In consideration of such acondition, research for supporting direct communication betweenterminals that have performed communication via a relevantinfrastructure is under progress. As part of the direct communication,obtaining synchronization between terminals in direct communication isimportant.

Therefore, a method and an apparatus for obtaining synchronization fordirection communication between devices in the case where aninfrastructure does not operate in an infrastructure-based communicationsystem are required.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method and an apparatus for obtainingsynchronization for direction communication between devices in the casewhere an infrastructure does not operate in an infrastructure-basedcommunication system.

Another aspect of the present invention is to provide a method and anapparatus for reducing interference between a terminal of an environmentwhere an infrastructure does not operate and a terminal that receives aservice from the infrastructure of a neighbor area in aninfrastructure-based communication system.

In accordance with an aspect of the present invention, a method forobtaining synchronization of a terminal for communication betweendevices when a base station collapses is provided. The method includes,when not receiving a preamble signal from the base station, maintaining,at the terminal, a first synchronization for a predetermined time usinga timer, transmitting, at the terminal, a first preamble signal at afirst synchronization point, performing, at the terminal, asynchronization procedure with at least one neighbor terminal based onthe first preamble signal, and after the synchronization procedure,performing communication with the at least one neighbor terminal.

In accordance with another aspect of the present invention, an apparatusof a terminal, for obtaining synchronization for communication betweendevices when a base station collapses is provided. The apparatusincludes a transmitter for, when a preamble signal is not received fromthe base station, allowing the terminal to maintain a firstsynchronization for a predetermined time using a timer and for allowingthe terminal to transmit a first preamble signal at a firstsynchronization point, and a controller for allowing the terminal toperform a synchronization procedure with at least one neighbor terminalbased on the first preamble signal and for allowing the terminal, afterthe synchronization procedure, to perform communication with the atleast one neighbor terminal.

In accordance with another aspect of the present invention, an apparatusof a terminal, for obtaining synchronization for communication betweendevices when a base station collapses is provided. The apparatusincludes a controller for, when a preamble signal is not received fromthe base station, monitoring a first preamble signal from a relevantneighbor terminal for a predetermined time, and a transmitter for, whenthe first preamble signal is not received from the relevant neighborterminal for the predetermined time, transmitting a second preamblesignal at an arbitrary point, wherein the controller performs asynchronization procedure with at least one neighbor terminal using thesecond preamble signal, and after the synchronization procedure,performs communication with the at least one neighbor terminal.

Other aspects, advantages and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a view illustrating a synchronization obtain scenario fordirection communication between devices in a case where aninfrastructure does not operate according to an exemplary embodiment ofthe present invention;

FIG. 2 is a flowchart illustrating a first terminal operation for directcommunication between devices in a case where an infrastructure does notoperate according to an exemplary embodiment of the present invention;

FIGS. 3A and 3B illustrate a flowchart of a second terminal operationfor direct communication between devices in a case where aninfrastructure does not operate according to an exemplary embodiment ofthe present invention; and

FIG. 4 is a block diagram illustrating a terminal apparatus for directcommunication between devices in the case where an infrastructure doesnot operate according to an exemplary embodiment of the presentinvention.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

Exemplary embodiments of the present invention relate to a method and anapparatus for obtaining synchronization for direct communication betweendevices in a case where an infrastructure does not operate in aninfrastructure-based communication system.

Exemplary embodiments of the present invention provide a method and anapparatus for obtaining synchronization for direct communication betweendevices in a case where an infrastructure does not operate. Moreparticularly, exemplary embodiments of the present invention provide amethod and an apparatus for obtaining, at devices, synchronizationbetween the devices using two types of synchronization signals (e.g.,preamble) when an infrastructure does not operate or collapses. In thefollowing description, the term “collapse” is used as a general term todesignate a mechanical, structural, electrical, functional, or any typeof failure or event that renders the infrastructure unable to performits intended duty.

In addition, exemplary embodiments of the present invention consider aproblem of reducing interference between a terminal performinginter-device communication due to an infrastructure disorder or collapseand a terminal performing communication via a neighbor infrastructureunder an environment where the infrastructure of a relevant area doesnot operate but the infrastructure of the neighbor area continues tooperate.

In an infrastructure-based communication system (for example, anInstitute of Electrical and Electronics Engineers (IEEE) 802.16 system),when a problem occurs at a base station that serves as theinfrastructure such that the base station does not operate, devices inthe cell serviced by the base station can no longer performcommunication via the base station. In this case, direct communicationbetween devices may be required. To enable direct communication betweendevices, synchronization between devices, obtained with asynchronization signal, is first required. The synchronization signalshould be designed such that the infrastructure may maintain the sameform as that before the infrastructure problem and such that aninfluence on the operation of a device may be minimized. Therefore, areference signal for obtaining synchronization between terminals has thesame structure as a preamble signal under the infrastructure. Forexample, in case of a system using the IEEE 802.16e standard, a preamblesignal is used. In case of a system using the IEEE 802.16m standard, aPrimary Synchronization Channel (P-SCH) is used. Considering an aspectof a structure, the same structure as a preamble of an existing systemis used but a preamble sequence used for obtaining synchronizationbetween terminals should be discriminated from that used in an existingsystem. In the case where a reserved sequence exists as in the case ofan IEEE 802.16m system, a portion of the reserved sequence may be used.In the case where a reserved sequence does not exist as in the case ofan IEEE 802.16e system, a sequence having a similar performance whilenot having an influence on an existing preamble performance may be newlydefined.

FIG. 1 is a view illustrating a synchronization obtain scenario fordirection communication between devices in a case where aninfrastructure does not operate according to an exemplary embodiment ofthe present invention.

Referring to FIG. 1, a plurality of base stations 100 and 110 existinside a service area, and each base station 100 and 110 is connectedwith a plurality of terminals inside its own cell area. Under thisinfrastructure environment, it is assumed that the base station 110 canlonger provide service due to a natural disaster or an unexpectedproblem and that the base station 100 normally operates. It is alsoassumed that terminals do not immediately lose synchronization even whenthe base station 110 collapses but maintain synchronization for apredetermined time via a timer.

When the base station 110 can no longer provide service, that is, thebase station 110 cannot transmit a synchronization signal or a preamblesignal, terminals located inside the cell area of the base station 110first attempt direct communication between devices in order to performcommunication without the use of the base station 110. For communicationbetween devices or direct communication between terminals, asynchronization procedure between devices or terminals should bepreceded.

For synchronization between devices, an exemplary embodiment of thepresent invention proposes that terminals obtain synchronization betweendevices using two preamble signals (or synchronization signals). The twopreamble signals include a first preamble signal transmitted by aterminal that maintains base station synchronization for a predeterminedtime when a base station collapses, and a second preamble signaltransmitted by a terminal that does not maintain base stationsynchronization after a predetermined time when a base stationcollapses. Here, the base station denotes timing at which a base stationtransmits a preamble. In other words, a point at which the firstpreamble signal is transmitted is the same as a point at which the basestation transmits a preamble, and a point at which the second preamblesignal is transmitted does not coincide with a point at which the basestation transmits the preamble. The preamble signal transmitted by thebase station is used as not only a synchronization signal but alsoinformation that can discriminate a base station. The first preamblesignal and the second preamble signal are used as not only asynchronization signal for communication between devices but alsoinformation indicating that a relevant base station has collapsed.

An exemplary synchronization procedure between devices is described.When the base station collapses, each terminal inside a cell of the basestation first tries to receive the first preamble signal. When notreceiving the first preamble signal for a predetermined time, each ofthe terminals tries to receive the second preamble signal. Whenreceiving the first preamble signal, each of the terminals synchronizesbetween devices based on the first preamble signal. Otherwise, each ofthe terminals synchronizes between devices based on the second preamblesignal. Even in the case where synchronization has been performed basedon the second preamble signal, the terminal tries to receive the firstpreamble signal every predetermined period to re-perform synchronizationbetween devices based on the first preamble signal. Though this, anexemplary embodiment of the present invention may minimize interferencebetween terminals inside a cell where a base station has collapsed andterminals inside a cell where a base station has not collapsed.

For example, when the base station 110 collapses, a portion of terminalslocated inside the cell of the base station 110 may obtain base stationsynchronization under influence of a neighbor base station 100 that hasnot collapsed, and the rest of the terminals may not obtain base stationsynchronization. Here, the terminals that have obtained the base stationsynchronization transmit the first preamble, and the terminals that havenot obtained the base station synchronization transmit the secondpreamble.

For example, a terminal 113 that has not maintained the base stationsynchronization transmits the second preamble signal in order tosynchronize with neighbor terminals. After transmitting the secondpreamble signal, the terminal 113 monitors whether a preamble signalfrom a base station or the first preamble signal from a neighbor basestation is transmitted. At this point, when receiving the first preamblesignal from a neighbor terminal 114 that maintains the base stationsynchronization inside a relevant region 130, the terminal 113 that hasnot maintained the base station synchronization may end communicationsynchronized using the existing second preamble signal, and may besynchronized using the received first preamble signal. When receiving apreamble signal from the base station 110, the terminal 113 that has notmaintained the base station synchronization ends communicationsynchronized using the existing second preamble signal and issynchronized using the received preamble signal of the base station 110.

Likewise, when receiving the first preamble signal from the neighborterminal 114 synchronized using the first preamble signal, terminals 112and 111 that have not maintained the base station synchronization may besynchronized using the first preamble signal. However, when notreceiving the first preamble signal from neighbor terminals, theterminals located inside a region 140 continue to maintainsynchronization based on the second preamble signal.

As described above, when a portion of terminals inside a cell where abase station has collapsed maintains base station synchronization eventhough terminals inside the cell where the base station has collapsed donot maintain the base station synchronization, the terminals that havenot maintained the base station synchronization may maintain the basestation synchronization afterward by a terminal that maintains the basestation synchronization. Therefore, since terminals inside the cellwhere the base station has collapsed are synchronized with terminals ofa neighbor base station, communication between terminals inside the cellwhere the base station has collapsed does not influence a neighbor cell.

Meanwhile, a portion of the terminals inside the cell where the basestation has not collapsed may transmit the first preamble, which may beperformed in the case where the terminal receives a structure signal ofa terminal of a neighbor cell (where an infrastructure has been damaged)or via a request of a relevant base station. That is, when not receivinga preamble signal from a base station for a predetermined time, theterminal tries to receive the first preamble or the second preamble.

FIG. 2 is a flowchart illustrating a first terminal operation for directcommunication between devices in a case where an infrastructure does notoperate according to an exemplary embodiment of the present invention.

Referring to FIG. 2, when it is determined that a preamble is notreceived from the base station 110 for at least a predetermined time instep 200, a terminal 114 that maintains base station synchronizationproceeds to step 202 to periodically broadcast the first preamble signalbased on a base station synchronization point.

In step 204, the terminal 114 tries to obtain synchronization with aneighbor terminal 113 based on the first preamble signal, and performscommunication with the neighbor terminal 113 in step 206. Here, thefirst preamble is transmitted at the same point as the point at whichthe preamble of the base station is transmitted based on a previous basestation synchronization point. Of course, the transmission of the firstpreamble is performed in accordance with a predetermined transmissionperiod of the preamble, which may be the same as the transmission periodof an existing system. That is, the terminal 114 continues to transmitthe first preamble according to the predetermined transmission period,obtains synchronization with a neighbor terminal based on thetransmitted first preamble, and starts a procedure for communicationbetween terminals.

In step 208, the terminal 114 determines if a preamble signal isreceived from the base station 110. When receiving a preamble signalfrom the base station 110, the terminal 110 proceeds to step 210. Whennot receiving the preamble signal from the base station 110, theterminal 110 returns to step 204.

In step 210, the terminal 114 ends direct communication with neighborterminals and performs a procedure for communication with the basestation 110.

Meanwhile, terminal 113 that has obtained synchronization via the firstpreamble tries to receive a base station preamble for each predeterminedtransmission period. When receiving the preamble of the base station,the terminal 113 tries to perform a procedure for communication with thebase station 110 again.

After that, the terminal 114 ends the procedure of an exemplaryembodiment of the present invention.

The method described above in relation with FIG. 2 under of the presentinvention may be provided as one or more instructions in one or moresoftware modules stored in the respective terminals.

FIGS. 3A and 3B illustrate a flowchart of a second terminal operationfor direct communication between devices in a case where aninfrastructure does not operate according to an exemplary embodiment ofthe present invention.

Referring to FIGS. 3A and 3B, when it is determined in step 300 that apreamble signal from a base station is not received for a predeterminedtime, for example due to collapse of the base station, the terminalproceeds to step 302 to monitor a first preamble signal.

In step 304, the terminal monitors and determines if the first preamblesignal that may be transmitted by a portion of neighbor terminals for apredetermined time is received. When receiving the first preamblesignal, the terminal proceeds to step 318. When not receiving the firstpreamble signal, the terminal proceeds to step 306.

When not receiving the first preamble signal, the terminal transmits thesecond preamble from an arbitrary point in step 306. The transmission ofthe second preamble is performed at a predetermined period, which may bethe same as the transmission period of the existing system.

After that, the terminal maintains synchronization with neighborterminals based on the second preamble signal according to apredetermined procedure in step 308.

The terminal performs a communication procedure with neighbor terminalsbased on the second preamble signal in step 310.

After that, the terminal monitors the preamble signal from the basestation or the first preamble signal from neighbor terminals in step312. It is determined in step 314 whether the preamble signal from thebase station or the first preamble signal from the neighbor terminals isreceived. When not receiving the preamble signal from the base stationor the first preamble signal from the neighbor terminals, the terminalreturns to step 308 to maintain synchronization based on the secondpreamble signal.

In contrast, when receiving the preamble signal from the base station,the terminal proceeds to step 316. When receiving the first preamblesignal from the neighbor terminals, the terminal proceeds to step 318.

The terminal ends communication with a neighbor terminal and performs acommunication procedure with the base station based on the received basestation preamble in step 316.

In step 318, the terminal is synchronized with a neighbor terminal basedon the first preamble signal, and performs a communication procedurewith a neighbor terminal using the first preamble signal in step 320.

In addition, the terminal monitors to determine if the preamble signalis received from the base station in step 322. When receiving thepreamble signal from the base station, the terminal proceeds to step324. When not receiving the preamble signal from the base station, theterminal returns to step 318.

The terminal ends direct communication with neighbor terminals andperforms a procedure for communication with the base station in step324.

That is, terminals that have not maintained base station synchronizationcontinue to transmit the second preamble signal according to apredetermined period to obtain synchronization with a neighbor terminalbased on the second preamble and start a procedure for communicationbetween terminals. Meanwhile, terminals that have obtainedsynchronization via the second preamble try to receive a base stationpreamble and the first preamble for each predetermined period. Whenreceiving the first preamble, the terminals transmit the first preambleat a point of receiving the first preamble. The operation at this pointis the same as the operation in FIG. 2. In addition, when receiving thebase station preamble, the terminals try to perform the procedure forcommunication with the base station again.

The method described above in relation with FIG. 3 under of the presentinvention may be provided as one or more instructions in one or moresoftware modules stored in the respective terminals.

FIG. 4 is a block diagram illustrating a terminal apparatus for directcommunication between devices in a case where an infrastructure does notoperate according to an exemplary embodiment of the present invention.

The terminal apparatus may be an electronic device such as, for example,a laptop, a smart phone, a net book, a mobile interne device, an ultramobile PC, a tablet personal computer, a mobile telecommunicationterminal, PDA having a camera and the like herein, just to name some ofthe possibilities.

Referring to FIG. 4, the terminal apparatus includes a receiver 400, acontroller 402, a transmitter 404, and a storage 406. Depending onrealization, the terminal apparatus may further include additionalfunctional blocks.

The receiver 400 converts a Radio Frequency (RF) signal received via anantenna into a baseband analog signal, and converts the analog signalinto sample data. In addition, the receiver 400 performs Fast FourierTransform (FFT) on the sample data to convert the sample data into datain a frequency domain, selects data of subcarriers which a user desiresto actually receive from the data in the frequency domain, and outputsthe same. In addition, the receiver demodulates and decodes the dataaccording to a predetermined Modulation and Coding Scheme (MCS) level,and outputs the same to the controller 402. For example, the receiver400 receives the preamble signal of the base station, the first preamblesignal or the second preamble signal from a neighbor terminal to performthe FFT operation.

The controller 402 controls an overall operation of the terminal. Inaddition, the controller 402 is provided with necessary informationwhile performing a protocol process from a relevant constituent (forexample, the receiver 400) of a physical layer, or generates a controlsignal to a relevant constituent (for example, the transmitter 404) ofthe physical layer. For example, the controller 402 controls asynchronization procedure for direct communication between devices whena base station collapses. Specifically, in case of maintaining basestation synchronization, the controller 402 transmits the first preamblesignal via the transmitter 404 based on the base station synchronizationand is synchronized with a neighbor terminal based on the first preamblesignal. Even when the controller 402 is synchronized with a neighborterminal based on the first preamble signal, the controller 402periodically monitors a preamble signal from the base station afterward.That is, the controller 402 controls the synchronization procedurebetween devices described in FIG. 2.

Meanwhile, in case of not maintaining base station synchronization, thecontroller 402 continues to transmit the second preamble signalaccording to a predetermined period to obtain synchronization with aneighbor terminal based on the second preamble and start a procedure forcommunication between terminals. Meanwhile, terminals that have obtainedsynchronization via the second preamble try to receive the base stationpreamble and the first preamble for each predetermined period. Whenreceiving the first preamble, the terminals transmit the first preambleat a point of receiving the first preamble. When receiving the basestation preamble, the terminals try to perform a procedure forcommunication with the base station again. That is, the controller 402controls the synchronization procedure between devices described inFIGS. 3A and 3B.

The storage 406 stores a preamble code corresponding to the firstpreamble signal used when the base station collapses and a preamble codecorresponding to the second preamble signal to provide the same to thecontroller 402 when needed.

The transmitter 404 encodes and modulates data from the controller 402according to a predetermined MCS level. In addition, the transmitter 404performs Inverse Fast Fourier Transform (IFFT) on a modulated symbol tooutput sample data (e.g., an Orthogonal Frequency Division Multiplexing(OFDM) symbol). For example, the transmitter 404 performs the IFFT onthe first preamble signal or the second preamble signal to transmit to aneighbor terminal.

Program instructions to perform a method described herein, or one ormore operations thereof, may be recorded, stored, or fixed in one ormore computer-readable storage media. The program instructions may beimplemented by a computer. For example, the computer may cause aprocessor to execute the program instructions. The media may include,alone or in combination with the program instructions, data files, datastructures, and the like. Examples of computer-readable media includemagnetic media, such as hard disks, floppy disks, and magnetic tape;optical media such as CD ROM disks and DVDs; magneto-optical media, suchas optical disks; and hardware devices that are specially configured tostore and perform program instructions, such as read-only memory (ROM),random access memory (RAM), flash memory, and the like. Examples ofprogram instructions include machine code, such as produced by acompiler, and files containing higher level code that may be executed bythe computer using an interpreter. The program instructions, that is,software, may be distributed over network coupled computer systems sothat the software is stored and executed in a distributed fashion. Forexample, the software and data may be stored by one or more computerreadable recording mediums. Also, functional programs, codes, and codesegments for accomplishing the example embodiments disclosed herein canbe easily construed by programmers skilled in the art to which theembodiments pertain based on and using the flow diagrams and blockdiagrams of the figures and their corresponding descriptions as providedherein. Also, the described unit to perform an operation or a method maybe hardware, software, or some combination of hardware and software. Forexample, the unit may be a software package running on a computer or thecomputer on which that software is running.

As described above, the first preamble signal transmitted at the samepoint as the transmission point of the base station preamble andtransmitted by the terminal when the base station does not operate, andthe second preamble signal transmitted at an arbitrary point regardlessof the transmission point of the base station preamble and transmittedby the terminal when the base station does not operate are used, so thatterminals inside a cell area of a base station that no longer provides aservice may maintain base station synchronization. In addition,interference between terminals inside the cell area of the collapsedbase station and terminals inside the cell area of a base station thathas not collapsed may be reduced.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method for operating a terminal, the methodcomprising: monitoring a synchronization signal from a base station;transmitting a first synchronization signal based on a synchronizationwith the base station; and performing communication with a neighborterminal which receives the first synchronization signal from theterminal.
 2. The method of claim 1, wherein the first synchronizationsignal indicates that the terminal fails to receive the synchronizationsignal from the base station.
 3. The method of claim 1, furthercomprising: monitoring the synchronization signal received from the basestation; if the synchronization signal is received from the basestation, performing synchronization based on the synchronization signal;and performing communication with the base station.
 4. The method ofclaim 1, wherein the first synchronization signal comprises a sequencewhich is different from sequences for the synchronization signaltransmitted by the base station.
 5. The method of claim 1, wherein thetransmitting the first synchronization signal comprises transmitting thefirst synchronization signal, if the terminal fails to receive thesynchronization signal from the base station.
 6. The method of claim 1,wherein the performing communication comprises performing communicationdirectly between devices.
 7. The method of claim 1, wherein the firstsynchronization signal indicates that the base station has collapsed. 8.The method of claim 1, wherein the synchronization signal from the basestation is used to as a synchronization signal and as information fordistinguishing the base station.
 9. A method for operating a terminal,the method comprising: monitoring a synchronization signal from a basestation; monitoring a first synchronization signal from a neighborterminal; transmitting a second synchronization signal, if the terminalfails to receive the first synchronization signal; and performingcommunication with a first neighbor terminal which receives the secondsynchronization signal.
 10. The method of claim 9, wherein the firstsynchronization signal indicates that the terminal fails to receive thesynchronization signal from the base station.
 11. The method of claim 9,further comprising: monitoring a synchronization signal from a basestation; and if the synchronization signal is received from the basestation, performing synchronization based on the synchronization signalto communicate with the base station.
 12. The method of claim 9, furthercomprising: monitoring a first synchronization signal from a secondneighbor terminal; and if the first synchronization signal is receivedfrom the second neighbor terminal, performing synchronization based onthe first synchronization signal to communicate with the second neighborterminal.
 13. The method of claim 12, further comprising: monitoring asynchronization signal from a base station; and if the synchronizationsignal is received from the base station, performing synchronizationbased on the synchronization signal to communicate with the basestation.
 14. The method of claim 9, further comprising: monitoring asynchronization signal from a base station and a first synchronizationsignal from a relevant neighbor terminal; and maintainingsynchronization with the first neighbor terminal, if the terminal failsto receive the synchronization signal from the base station and thefirst synchronization signal from the neighbor terminal.
 15. The methodof claim 9, wherein the transmitting a first synchronization signalcomprises transmitting the first synchronization signal, if the terminalfails to receive the synchronization signal from the base station. 16.The method of claim 9, wherein the performing communication comprisesperforming communication directly between devices.
 17. The method ofclaim 9, wherein the first synchronization signal indicates that thebase station has collapsed.
 18. The method of claim 9, wherein thesynchronization signal from the base station is used to as asynchronization signal and as information which is used fordistinguishing the base station.
 19. The method of claim 9, wherein thesecond synchronization signal comprises a sequence which is differentfrom sequences for the synchronization signal transmitted by the basestation.
 20. The method of claim 9, wherein the first synchronizationsignal is transmitted based on a synchronization with the base station.21. The method of claim 9, wherein the second synchronization signalindicates that the terminal fails to receive the synchronization signalfrom the base station.
 22. An apparatus for a terminal, the apparatuscomprising: a controller configured to monitor a synchronization signalfrom a base station; and a transmitter configured to, transmit a firstsynchronization signal based on synchronization with the base station,wherein the controller is further configured to perform communicationwith a neighbor terminal which receives the first synchronization signalfrom the terminal.
 23. The apparatus of claim 22, wherein the firstsynchronization signal indicates that the terminal fails to receive thesynchronization signal from the base station.
 24. The apparatus of claim22, wherein the controller is further configured to: monitor thesynchronization signal received from the base station; performsynchronization based on the synchronization signal if thesynchronization signal is received from the base station; and performcommunication with the base station.
 25. The apparatus of claim 22,wherein the first synchronization signal comprises a sequence which isdifferent from sequences for the synchronization signal transmitted bythe base station.
 26. The apparatus of claim 22, wherein the transmittertransmits the first synchronization signal based on a synchronizationwith the base station if the terminal fails to receive thesynchronization signal from the base station.
 27. The apparatus of claim22, wherein the controller is further configured to performcommunication directly between devices.
 28. The apparatus of claim 22,wherein the first synchronization signal indicates that the base stationhas collapsed.
 29. The apparatus of claim 22, wherein thesynchronization signal from the base station is used to as asynchronization signal and as information which is used fordistinguishing the base station.
 30. An apparatus for a terminal, theapparatus comprising: a controller configured to monitor asynchronization signal from a base station, and, to monitor a firstsynchronization signal from a neighbor terminal, if the terminal failsto receive the synchronization signal from the base station; and atransmitter configured to transmit a second synchronization signal,wherein the controller is further configured to perform communicationwith the at least one neighbor terminal which receives the secondsynchronization signal.
 31. The apparatus of claim 30, wherein the firstsynchronization signal indicates that the terminal fails to receive thesynchronization signal from the base station.
 32. The apparatus of claim30, wherein the controller is further configured to monitor asynchronization signal from a base station; and to performsynchronization based on the synchronization signal to communicate withthe base station if the synchronization signal is received from the basestation.
 33. The apparatus of claim 30, wherein the controller isfurther configured to monitor a first synchronization signal from asecond neighbor terminal, and to perform synchronization based on thefirst synchronization signal to communicate with the second neighborterminal if the first synchronization signal is received from the secondneighbor terminal.
 34. The apparatus of claim 33, wherein the controlleris further configured to monitor a synchronization signal from a basestation, and, to perform synchronization based on the synchronizationsignal to communicate with the base station if the synchronizationsignal is received from the base station.
 35. The apparatus of claim 30,wherein the controller is further configured to monitor asynchronization signal from a base station and a first synchronizationsignal from a neighbor terminal, and to maintain synchronization withthe first neighbor terminal if the terminal fails to receive thesynchronization signal from the base station and the firstsynchronization signal from the neighbor terminal.
 36. The apparatus ofclaim 30, wherein the second synchronization signal comprises a sequencewhich is different from sequences for the synchronization signaltransmitted by the base station.
 37. The apparatus of claim 30, whereinthe first synchronization signal is transmitted based on asynchronization with the base station.
 38. The apparatus of claim 30,wherein the second synchronization signal indicates that the terminalfails to receive the synchronization signal from the base station. 39.The apparatus of claim 30, wherein the transmitter transmits the firstsynchronization signal based on a synchronization with the base stationif the terminal fails to receive the synchronization signal from thebase station.
 40. The apparatus of claim 30, wherein the controller isconfigured to perform communication directly between devices.
 41. Theapparatus of claim 30, wherein the first synchronization signalindicates that the base station has collapsed.
 42. The apparatus ofclaim 30, wherein the synchronization signal from the base station isused to as a synchronization signal and as information which is used fordistinguishing the base station.